1. Field of The Invention
The invention relates to a method of displaying graphics subdivided into areas with a foreground and a background, each of which is area-wise switchable to a selectable color, and in which graphics areas are transparently switchable to at least a different display plane.
2. Description of The Related Art
Such graphics displays are particularly used for teletext in television apparatuses. The teletext has a foreground which mostly consists of displayed characters or other graphics. The background is usually a colored area which may also have different colors in different parts of this area. Moreover, it is known for such teletext displays to be able to transparently switch the foreground and/or the background. This means that in the respective transparently switched areas of the foreground and/or the background, it is not this foreground or background which is displayed but a different display plane. This different display plane may be, for example a video image which becomes visible in the transparently switched areas.
In the method of transparent switching known in the state of the art, additional bits are stored as transparent bits in the display memory. It is also possible to use a specific color which is stored in the display memory for the transparent switching. In both cases, additional memory sites are required, which are reserved for the transparent switching. Either attribute bits are reserved for the transparent display in the display memory, or memory sites are reserved in a color memory, these sites being assigned to the color at which transparent switching should actually take place.
It is an object of the invention to provide a method of transparent switching in such graphic applications, in which no additional memory space is required for the transparent switching.
According to the invention, this object is achieved in that a foreground color selected in an area and a background color selected in the same area are compared area-wise, and in that those areas of the foreground and/or the background of the graphics are transparently switched in which a predetermined constellation of the selected foreground color and the selected background color is determined.
In the method according to the invention, no additional memory space is required for the transparent switching of corresponding areas of a foreground and/or a background of a graphics display, because no additional bits or no specific colors are used for the transparent switching. Instead, the constellation of the selected foreground color and the selected background color is accentuated. Given combinations of foreground color and background color may be predetermined, at whose occurrence the corresponding areas are switched transparently. Advantageously, constellations of foreground color and background color which do not occur anyway or are not useful, i.e., constellations not used for the normal graphics display, may be selected for this purpose. This comparison is made individually for the separate areas of the graphics display so that transparent switching takes place or does not take place area-wise.
It is thereby achieved that memory bits which are necessary anyway for the colors of the foreground and/or the background, can be additionally used for the transparent switching.
The embodiment according to the invention, wherein those areas of the foreground and/or the background of the graphics are switched transparently in which the same color is selected for the foreground and the background, utilizes the fact that an identical foreground and background color in the graphics display or a teletext display would not be recognizable anyway and utilizes this constellation for transparent switching. For the graphics display, there is no limitation of possible combinations of foreground and background colors because graphics with identical foreground and background colors cannot be recognized anyway.
The further embodiment according to the invention, wherein changes of the selected foreground color and the selected background color between a first area and a neighboring, second area are determined, and in that, dependent on the foreground and background colors selected for the first area and on the changes of the selected colors between the second and the first area a decision is made on whether the foreground or the background is switched transparently, further improves the method to the extent that the constellation of foreground and background color is not only tested statically in the separate areas, but the change of foreground and background colors from a second area to a first area is also tested. Dependent on the change of the foreground and background colors from the second area to the first area, a decision can be made on whether only the foreground, only the background, both or none of the two are switched transparently. Thus, the change of the foreground and background colors is taken into account as an additional criterion for the transparent switching, with a decision being possible for the foreground and the background individually.
Further embodiments of the method describe advantageous constellations for the transparent switching, with the change of selected foreground and/or background color being taken into account for the decision.
In a further embodiment according to the invention, wherein the graphics are videotext for whose symbols foreground colors are selectable, the videotext symbols being displayable on a background whose color is selectable as a background color, and in that a picture signal is displayable on the other display plane, the method according to the invention may be particularly used advantageously for teletext in which it is generally desired to implement a transparent function which can be realized in accordance with the invention without any additional memory space.
These and other aspects of the invention will be apparent from and elucidated with reference to the embodiments described hereinafter.